Thin layer composite unimorph ferroelectric driver and sensor

ABSTRACT

A method for forming ferroelectric wafers is provided. A prestress layer is placed on the desired mold. A ferroelectric wafer is placed on top of the prestress layer. The layers are heated and then cooled, causing the ferroelectric wafer to become prestressed. The prestress layer may include reinforcing material and the ferroelectric wafer may include electrodes or electrode layers may be placed on either side of the ferroelectric layer. Wafers produced using this method have greatly improved output motion.

This application is a continuing application of commonly-owned patentapplication Ser. No. 08/416,598, filed Apr. 4, 1995, now U.S. Pat. No.5,632,841.

ORIGIN OF THE INVENTION

The invention described herein was made by employees of the UnitedStates Government and may be used by and for the Government forgovernmental purposes without the payment of any royalties thereon ortherefor.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates generally to ferroelectric devices, andmore particularly to ferroelectric devices providing large mechanicaloutput displacements.

2. Discussion of the Related Art

Prior art methods include ‘Rainbow’ piezoelectric actuators and sensors,more conventional piezoelectric actuators and sensors, andelectro-magnetic actuators.

Conventional piezoelectric actuators exhibit limited mechanicaldisplacements. The output of conventional piezoelectric devices islimited by the material's basically low piezoelectric displacementconstant. Thus conventional devices of reasonable thickness (i.e. on theorder of a few millimeters) offer only micrometer-sized mechanicaloutput motion. ‘Rainbow’ actuators, ‘Moonies’, unimorphic, and bimorphicpiezoelectric actuators exhibit greater mechanical output motion.However even the thinnest ceramic wafers, which exhibit the maximumobserved output motion, provide a displacement limited to approximately1 mm of motion in the z-direction for a device that is 3-4 cm long.Additionally ¼ mm thick ceramic devices are extremely brittle andfragile so that they are prone to breakage and require special handling.Previous methods of forming ‘Rainbow’ actuators include an additionalchemical reduction process which releases lead vapors from the waferinto the atmosphere.

It is accordingly an object of the present invention to provide aferroelectric actuator with improved mechanical displacement.

It is another object of the present invention to provide a ferroelectricactuator with improved durability.

It is another object of the present invention to provide a ferroelectricactuator with improved machinability.

It is another object of the present invention to provide a method forproducing a ferroelectric actuator which is more environmentally safethan previous methods.

It is yet another object of the present invention to accomplish theforegoing objects in a simple manner.

Additional objects and advantages of the present invention are apparentfrom the drawings and specification which follow.

SUMMARY OF THE INVENTION

According to the present invention, the foregoing and additional objectsare obtained by providing a method for producing ferroelectric devices.First, a mold is selected for the device. A prestress layer is placed onthe mold and a ferroelectric layer is placed on top of the prestresslayer. These layers are bonded together by heating and then cooling theassembled device. The prestress layer may be an adhesive and may includereinforcing material. The ferroelectric layer may be a piezoelectricmaterial, a piezostrictive material or a composite. The ferroelectriclayer includes surface electrodes which may be applied by including anelectrode layer on either side of the ferroelectric layer prior toheating the assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the preferred embodiment prior tobonding the layers;

FIG. 2 is a cross sectional view of the preferred embodiment aftercooling of the layers;

FIG. 3 is a cross sectional view of an alternate embodiment of thepresent invention;

FIG. 4 is a cross sectional view of an alternate embodiment of thepresent invention;

FIG. 5 is a cross sectional view showing the manufacturing process ofthe present invention;

FIG. 6 is perspective view showing an alternate embodiment of thepresent invention;

FIG. 7 is a top view showing a plurality of prestressed wafers connectedto form a larger wafer; and

FIG. 8 is a side view showing three of the prestressed wafers in astacked configuration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a piezoelectric device 10 according to the presentinvention prior to being processed. The device includes four layers, apiezoelectric layer 12, a prestressing layer 14 and two electrode layers18 a and 18 b. The piezoelectric layer 12 can be made from a disk ofpiezoelectric material commercially available from Aura Ceramics (C3900material) or Morgan Matrox. Alternatively, this layer can be made frompiezoelectric material that was first ground to a fine powder andsubsequently consolidated into a layer by compression bonding the powderwith an adhesive such as a polyimide, as shown in “Tough, Soluble,Aromatic, Thermoplastic Copolyimides”, Ser. No. 08/359,752, filed Dec.16, 1994. Note that in the latter approach, the adhesive binder makes upa very small fraction, typically 5 percent by weight, of the finishedpiezoelectric layer 12. This latter approach is attractive since therequired bonding operation can simply be performed simultaneously withother needed bonding operations discussed in the next paragraph. Inaddition to piezoelectric materials, other ferroelectric materials,including piezostrictive materials may be used to form this layer.

The prestressing layer 14 can be made of a mechanically strong adhesivesuch as a polyimide. Thermoplastics, thermosets and braze alloys mayalso be used for this layer 14. Additionally, multiple prestress layers14 may be used to provide increased prestress. The adhesive iswet-coated or a thin film is melted onto one surface of thepiezoelectric layer 12 and then bonded to it at an elevated temperaturewhich is dependent on the adhesive being used and allows the material toundergo cure, drying, and/or melt flow. The layer of adhesive thusformed is typically twelve microns thick, but can range in thicknessfrom a few microns to several mm. Bonding of the layers occurs at a hightemperature, which depends upon the adhesive but is typically 200-350°C., so that when the two-layer composite matrix cools to roomtemperature, the differential thermal compression rates of the layersautomatically impart the desired mechanical prestress into the layers,as shown in FIG. 2. If desired, the prestressing layer 14 of adhesivecan be reinforced primarily to allow augmenting the level of prestress,but also for mechanical toughness and decreased hysteresis. Toaccomplish this, a thin layer of reinforcing material 16 is fused orbonded onto (FIG. 3), or into (FIG. 4), the prestressing layer 14.Examples of reinforcing materials include, but are not limited to,plastics, ceramics, metals and combinations of these materials such asaluminum sheet stock and carbon fibers. Bonding of the reinforcingmaterial 16 can occur simultaneously with the bonding of thepiezoelectric to the prestressing layer.

The adhesive layer allows the thin ceramic wafer to be cut to shapewithout chipping or fracturing using conventional cutting tools likescissors and pattern cutters allowing tailor-made shapes rather thanmolded shapes. This method enables techniques to be used which allow thepattern building of 3-dimensional objects from the consolidation of the2-dimensional flat ceramic shapes. These layers can also offeradditional environmental protection which allows these devices to beused in harsh conditions. If the adhesive layer used is a gooddielectric, the chances of internal and external arcing due to theapplied voltage are reduced.

In one embodiment, the piezoelectric device 10 according to the presentinvention contains two electrodes 18 a and 18 b. The electrodes 18 a and18 b can be of the more conventional vacuum-deposited metal type, andcan be applied onto the piezoelectric layer 12 prior to application ofthe prestressing layer 14. Alternatively, the electrodes can be agraphite or metal-plus-adhesive mixture such as silver epoxy, which isan excellent conductor. This alternate technique has the advantage thatthe conductive adhesive mixture can be coated onto the piezoelectriclayer 12 and subsequently bonded to the piezoelectric layer 12,simultaneous with the bonding of the prestressing 14 and piezoelectriclayers 12. Multiple or patterned electrodes may also be used ifnecessary for the desired application.

The above teachings, may be combined to simplify the manufacture ofpiezoelectric devices. Complete devices can be produced by assemblingseparate layers of different materials, such as the appropriate mixturesof adhesively coated powdered piezoelectric material plus adhesive forthe piezoelectric layer, conductive adhesive for the electrodes, and theadhesive by itself or as reinforcement for the prestressing layer,followed by a single high-temperature bonding operation as described in“Tough, Soluble, Aromatic, Thermoplastic Copolyimides”, Ser. No.08/359,752, filed Dec. 16, 1994.

Provisions should be made during the manufacturing process to ensurethat the finished piezoelectric device has its prestressing layer intension which places the piezoelectric material in the desiredcompression. The amount of prestress in the piezoelectric material canbe tailored during manufacture in order to maximize the output motionand efficiency of the final device. The material layers may be formed ona curve-shaped mold.

A description typical of fabricating a prestressed device 10 by hand isprovided here and shown in FIG. 5. A molding surface 20 is selected forthe amount of curvature needed to provide the desired prestress. Theprestress reinforcing layer 16 of aluminum foil is then placed on top ofthe molding surface 20. Next the adhesive prestress layer 14 made from apolyimide as described in “Tough, Soluble, Aromatic, ThermoplasticCopolyimides”, Ser. No. 08/359,752, filed Dec. 16, 1994 is placed on topof the reinforcing layer 16. The electrode layer 18 b of silver isvacuum deposited on the lower surface of the piezoelectric wafer 12(this step is unnecessary if pre-electroded piezoelectric wafers arebeing used). The piezoelectric wafer 12 is placed on top of the adhesiveprestress layer 14. Finally, an electrode layer 18 a of silver is vacuumdeposited on the upper surface of piezoelectric wafer 12, if necessary.A sheet of high temperature material 22, such as Kapton® (DuPont), isplaced over the stack and is sealed using high temperature bagging tape24 to produce a vacuum bag. The assembly is placed into an oven and theair in the Kapton® bag 22 is evacuated through vacuum port 26. The ovenis heated to 300° C. to melt the adhesive and bond the assembly. Uponcooling, the assembly undergoes further prestressing, and the resultingpiezoelectric device is removed from the vacuum bag and mold.

Although the ferroelectric wafers are typically poled as received fromthe vendor, they must be repoled following thermal treatment in theprestress process. The poling is done at an elevated temperature with aDC voltage sufficient to induce dipole orientation. After poling, thewafer is cooled to room temperature in the presence of the electricfield to preserve the induced orientation. The DC field strengthemployed in the polarization is selected to obtain optimum polarizationwithout exceeding the field at which dielectric breakdown occurs in thematerial at a given poling temperature.

The amount and type of input voltage per unit of deflection, motion,force and output voltage, current, or power conversion can be adjustedby varying the thickness and/or number of layers of the piezoelectric,the number of layers and/or thickness of the prestress layer, theprestress material, the piezoelectric composition and the curvature andshape of the molding surface. By varying the curvature of the mold, theprestress imparted to the finished piezoelectric device is varied. Byvarying the thickness or number of prestress material layers or byvarying the material used, the output motion and mechanical force canalso be varied. During processing, the piezoelectric and prestressinglayers move with respect to each other and upon cooling bond togetherwith additional prestress. This method of making devices has shownsubstantial increase of output motion of otherwise identicalpiezoelectric devices.

A cylindrical bender mode may be emphasized by prestressing in only onedirection which can be done by bending the layers over a cylindricalmolding surface during the heating cycle. On cooling, the prestressinglayer 14, being under the piezoelectric layer 12 has a tighter radius ofcurvature and prestresses more in one direction thus forming the bender.These cylindrical mode benders are typically shapes other than circularas shown in FIG. 6.

A number of individual, polygon-shaped piezoelectric devices 28 can begrouped into a large-area mosaic by joining their appropriate edges asshown in FIG. 7. One useful method for edge attachment is the use of asingle reinforcing layer that covers the entire mosaic.

Certain applications may require more mechanical output force than canbe provided by a single device 10. Two or more devices 10 can then beused in an efficient tandem arrangement by uniting their dome-likeshapes in a ‘stacked-spoons’ configuration. FIG. 8 shows three devicesin this stacked configuration. To allow unimpeded bending of theindividual devices during operation the devices can be bonded to eachother using a compliant layer 30, such as an elastomer, i.e. siliconerubber, which allows one layer to move relative to the other. In such anactuator stack, the individual devices 10 remain electrically isolatedfrom each other; one or more of the devices 10 can act as a motionfeedback sensor.

When made using a matrix composite fabrication method shown in “Tough,Soluble, Aromatic, Thermoplastic Copolyimides”, Ser. No. 08/359,752,filed Dec. 16, 1994, large flexible sheets may be produced for use inlow-frequency actuator applications (i.e. noise canceling devices orloud speakers). This can be accomplished by using large flat molds forconsolidation or a continuous roll process. Molded parts can be bondedtogether by heating them to soften and/or cure the binder adhesive whilepressing them together.

Ferroelectric devices made from the present method can be used in pumps,valves, brakes, motors, sensors, actuators, optics, acoustictransducers, and active structures.

Many improvements, modifications, and additions will be apparent to theskilled artisan without departing from the spirit and scope of thepresent invention as described herein and defined in the followingclaims.

What is claimed is:
 1. An electroactive device providing largemechanical output displacements, comprising: a layered structure havinga prestressing layer having a convex surface, and a piezoelectric layerhaving a concave surface, the convex surface of the prestressing layerbeing bonded onto the concave surface of the piezoelectric layer suchthat the prestressing layer is in tension and imparts a prestress on thepiezoelectric layer such that the piezoelectric layer is in compression,wherein the prestressing layer and the piezoelectric layer are distinctfrom one another.
 2. The device of claim 1, wherein the prestressinglayer includes reinforcing material.
 3. The device of claim 1, whereinthe piezoelectric layer includes surface electrodes.
 4. The device ofclaim 1, further comprising: an electrode layer placed between theprestressing layer and the piezoelectric layer; and an electrode layerplaced on top of the piezoelectric layer.
 5. The device of claim 1,wherein the prestressing layer is an adhesive.
 6. The device of claim 1,where the piezoelectric layer is a ferroelectric material.
 7. The deviceof claim 1, wherein the piezoelectric layer is a piezorestrictivematerial.
 8. The device of claim 5, wherein the adhesive is a polyimide.